1. Field of the Invention
The present invention relates to a document reading apparatus and a document reading method for reading a document including a tabbed sheet while conveying the document.
2. Description of the Related Art
There is known a digital copying apparatus having an electronic sorting function that involves reading images of a plurality of documents, temporarily storing the images in an image memory, and repeatedly reading out and printing out the images in an arbitrary order for a desired number of copies. As this type of digital copying apparatus, there is known a digital copying apparatus including an automatic document feeder (ADF). In the ADF, a fed document is nipped by a roller pair, and is conveyed to a predetermined reading position. In this case, when a leading edge of the conveyed document enters the nip of the roller pair, a shock may occur in the document. The shock may also occur when a trailing edge of the document passes through the roller pair. Due to the shock, the position of the document is misaligned, thus leading to a problem of image reading failure such as color misregistration.
Further, there is a demand to read an image of a tabbed sheet. The tabbed sheet refers to a sheet to be used as a divider in a sheet bundle formed of a plurality of standard-size sheets, and to a non-standard-size sheet including a tab (protruding index) at an arbitrary position on one edge of the sheet. In general, thick paper is used for the tabbed sheet. Therefore, the above-mentioned shock becomes more significant.
As a related art for solving the problem described above, for example, there is known an image reading apparatus disclosed in U.S. Pat. No. 7,753,355. In this image reading apparatus, first and second roller pairs provided on upstream and downstream sides of the reading position are separable, respectively. That is, nipping forces of the first and second roller pairs are changed depending on the type of sheet, such as thick paper and thin paper. Thus, influence of the above-mentioned shock is mitigated.
In the image reading apparatus disclosed in U.S. Pat. No. 7,753,355, a timing of the separation when the document passes through each roller pair is determined with reference to a timing of trailing edge detection for the document using a sensor. When a timing of leading edge detection for the document is set as the reference, in a case where a plurality of documents different in length in a conveyance direction are to be read, it is necessary to detect information on the lengths of all the documents in advance.
In a case of using the tabbed sheet, however, there is a problem that cannot be solved even by the image reading apparatus disclosed in U.S. Pat. No. 7,753,355. This problem resides in that an installation position of the sensor is fixed but a position of the tab in a direction orthogonal to the document conveyance direction is not necessarily constant. This problem is described with reference to FIG. 17. In the example of FIG. 17, a tab T1 of a document A to be conveyed is located at an edge detection position of the sensor. Therefore, a document length of the document A is detected with reference to a trailing edge of the tab T1. On the other hand, a tab T2 of a document B is not located at the edge detection position of the sensor. Therefore, a document length of the document B is detected with reference to a trailing edge of the document B. As described above, the timings of trailing edge detection for the document A and the document B differ depending on the positions of the respective tabs. Therefore, the above-mentioned shock problem remains unsolved.
Note that, there is also known a related art for detecting the tab by arranging a plurality of sensors in the direction orthogonal to the document conveyance direction, to thereby detect the position of the trailing edge of the document irrespective of the position of the tab. In this related art, however, the plurality of sensors are necessary, resulting in cost increase. Further, there is also known a related art for controlling the timing of separation of rollers based on a document size and a detection result of the document trailing edge. In this related art, when the document size is the same, the rollers are separated at the same timing irrespective of presence and absence of the tab, and hence the above-mentioned shock problem still remains unsolved.